A conventional control apparatus for a multi-phase rotary machine controls a multi-phase rotary machine by controlling ON/OFF switching of a plurality of switching elements. In such a control apparatus for a rotary machine, it is proposed by the following patent documents to allow the rotary machine to continue its operation even when a failure occurred in a part of the switching element.    Patent document 1: US 2007/0176577 A1 (WO 2005/091488 A1)    Patent document 2: JP 2005-304119A
In patent document 1, when the OFF failure (a failure in which a switching element is always held in a non-conductive state) occurs in one of the switching elements, the control apparatus continues to drive the rotary machine to operate. However, when the ON failure (a failure in which a switching element is always held in a conductive state) occurs in one of the switching elements, the control apparatus does not continue to drive the rotary machine.
In patent document 2, when the ON failure occurred in one of the switching elements, the control apparatus continues to drive the rotary machine by one of a plurality of inverters. When the ON failure occurred in one of the switching elements, a coil in each phase of the rotary machine and a power source or a ground become conductive with each other. When the rotary machine is driven in such a state, an induction voltage is generated in the rotary machine. Due to this induction voltage, a torque which resists the driving (hereinafter referred to as brake torque) is generated in the rotary machine. The brake torque is increased as a rotational speed of the rotary machine increases. Further, since the induction voltage is a sinusoidal wave voltage, the brake torque changes with an electrical angle. Accordingly, when the rotary machine is continued to be driven by the non-failure inverter, the output torque of the rotary machine also changes with the change in the brake torque.